An up-conversion phosphor is a material capable of radiating light higher in energy than excitation light.
An up-conversion phosphor can use a low energy light source, and hence is expected to cope with applications in various fields; however, because a phosphor usually radiates light lower in energy than the excitation light (down-conversion), in order to give rise to up-conversion phenomenon, it is necessary to involve, for example, excited-state absorption, multi-photon absorption and energy transfer.
Accordingly, various materials have been investigated, and at the same time, various investigations and proposals have been performed for the purpose of enhancing light emission efficiency.
For example, some relevant reports are listed below: a report on phosphor microparticles represented by the compositional formula, (R1-x,Erx)2O3 (R is at least one of Y, La, Gd and Lu, and x is given in terms of molar quantity and satisfies the relation, 0.001≦x≦0.20), and is caused to exhibit up-conversion light emission by the light having a wavelength falling within a range from 500 nm to 2000 nm (see Patent Literature 1); a report on a visible up-conversion light emission from Y2O3:Eu3+,Yb3+ (see Non Patent Literature 1); a report on the up-conversion properties of Er3+—Yb3+:NaYF4 including Yb3+ having a wide concentration range (see Non Patent Literature 2); a report on the up-conversion fluorescence from Er3+ in nanocrystals Y2Si2O7:Er3+ and Y2Si2O7:Yb3+,Er3+ (see Non Patent Literature 3); a report on the up-conversion fluorescence from infrared to visible light in an Er3+/Yb3+ added titanate glass prepared by a method using no vessel (see Non Patent Literature 4); a report on the technique to produce up-conversion nanoparticles by irradiating a target (made of a fluorescent material having up-conversion properties) in a liquid with a laser light (see Patent Literature 2); and a report on blue up-conversion from near infrared, in colloidal BaYF5:Tm3+,Yb3+ nanocrystals (see Non Patent Literature 5).